mask panties
This tag describes the situation where a mouth mask (such as a surgical mask) is worn over the crotch as a substitute for panties. This is shown either by stretching the ear loops of the mask to use as leg holes (see Danbooru post #2582922), or more commonly by tying, interlocking, or otherwise binding multiple masks together to be worn as a larger outfit often incorporating a mask bikini (see Danbooru post #4048975, Danbooru post #4329843). Ironically, the character involved may or may not additionally being wearing a mask properly over their face. This theme developed in response to the coronavirus pandemic of 2020 where mask usage was brought into stark focus.
For the converse of this tag, where panties are being worn as a mask, use the tag panty mask.